1. Field of the Invention
The present invention relates generally to semiconductor processing and test equipment, and more particularly to a method and system for identifying semiconductor fabrication contaminants by subjecting wafers to temperature excursions at a low pressure, and by analyzing the compounds which are released using gas chromatography and/or mass spectrometry.
Semiconductor wafers are highly sensitive to contaminants introduced into the wafer structure during fabrication. As the processes involved in fabrication can be highly complex, it is desirable to specifically identify any sources of contamination during fabrication, rather than searching for contaminants after fabrication is complete and the finished semi-conductor devices fail to perform as desired.
Recently, mass spectrometers have been used to test for contaminants within deposition chambers. Mass spectrometers are particularly well adapted for sensing and quantifying a limited number of atomic or molecular compounds. For example, mass spectrometers now sense H.sub.2, H.sub.2 O, N.sub.2, O.sub.2 and CH.sub.x present in plasma vapor deposition chambers. Hence, by attaching a mass spectrometer to the evacuated plasma deposition chambers, the spectrometer can detect vacuum leaks as unacceptable amounts of nitrogen, oxygen, or water in the process chamber environment. Additionally, mass spectrometers may be able monitor for a few known contaminants within a semi-conductor process chamber, such as contaminants from photo-resist residue and the like.
Unfortunately, identifying an unknown contaminant from an unknown contamination source remains problematic. Often, the specific compound causing contamination or failure of the semi-conductor device is not known. Semi-conductor devices are being built with more and more layers having a wide variety of layer materials, and the interaction of all of the compounds to which the wafer is exposed can be difficult to predict. In an attempt to provide a mechanism for identifying contaminants, it has recently been proposed to heat small pieces of the completed wafer in an evacuated chamber. The temperature is increased in a series of steps, with mass spectrometry being applied at each step to quantify a few atomic species or simple compounds which are released by the wafer at each temperature. By studying the relative amounts of these target species which are released over time, the make-up of the contaminating compound can sometimes be derived. This process is called "trending".
Trending suffers from several distinct disadvantages. First, the wafer is destroyed in the testing process, both by being broken into small pieces, and by heating at a number of elevated temperatures for a significant amount of time. Second, trending can only indicate a limited number of contaminant compounds, and only where the composition of those compounds is known before the test is initiated. As the contaminants themselves are often broken into atomic or simple molecular substructures, precise identification of a complex or unknown compound is difficult and/or impossible. Trending also requires skilled analysts having a knowledge of the contaminant compounds, including their boiling points, their disassociation pathways, and the like. Even for these specialists, it can be difficult to determine the specific contaminant species. For example, when monitoring the quantity of hydrogen released at various temperatures, it can be difficult to determine whether the hydrogen is released from water, a complex photo-resist, or any of thousands of other compounds. Where more than one contaminant compound is present, identification of each compound using the trending process is particularly problematic.
For the above reasons, it would be desirable to provide improved methods and systems for testing of semi-conductor wafers. Preferably, such systems and methods would allow identification and quantification of compounds and contaminants, even where the presence of the compound is not suspected. It would be ideal to provide non-destructive wafer test methods for monitoring wafer quality without decreasing the total quantity of wafers produced.
2. Description of the Background Art
U.S. Pat. No. 5,152,176, describes a process and apparatus for determining the quantity of impurities in a gas by chromatography in a gas phase. U.S. Pat. No. 5,522,918, describes a method for analyzing organic substances within a clean room by sub-cooling a semi-conductor substrate to trap volatile organic substances on the substrate.